Energy saving is one of the most important requirements in any system or device. For this purpose, the AC grid is being replaced by local DC grids in some applications.
One advantage of DC grids is that they offer the opportunity to deploy compact low cost highly reliable linear drivers for LED based lighting.
In practice, to reduce energy consumption, dimming control technologies have also been employed in LED drivers. A wide dimming range is needed for accommodating different operating conditions. Traditionally, there are different categories of dimming methods, including analog dimming and pulse width modulation (PWM) dimming.
In analog dimming, current amplitude adjustment inherently gives rise to color temperature variations. The use of analog dimming is not recommended in applications where the color of the LED is critical.
In PWM dimming, the average amount of the LED current used for driving the LED light is usually determined based on the pulse width and period of a PWM signal. When the dimming level is decreased and the on-cycle of the LED current is shortened, human eyes can perceive a flicker of light. This restricts the dimming range (in particular the threshold for the minimum pulse width) to achieve predictable and acceptable performance from the LED device. In addition, the efficiency of LED drivers is in the range of 85-90% at low dimming levels.
Various control methods for LED dimming are known to control the color shift and flicker. In general lighting applications (indoor and outdoor lighting), the issue of efficiency and flicker is more important than the issue of color shift. Thus, analog dimming can be considered in indoor and outdoor lighting applications. Furthermore in an application using a DC grid, which is a local grid, the bus voltage variation (±2%) is usually less than for a mains AC application (±15%) hence simple low cost highly reliable linear drivers for LED based lighting can be used, as shown in FIG. 1.
The driver circuit shown in FIG. 1 comprises a DC input 10 such as a bus voltage, a load in the form of a string 12 of LEDs and a linear LED driver 14.
The linear LED driver 14 provides a resistance which varies in accordance with the load, resulting in a constant output voltage. It functions as a regulating device which is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage, and continually dissipating the difference between the input and regulated voltages as waste heat. Because the regulated voltage of a linear regulator must always be lower than input voltage, efficiency is limited and the input voltage must be high enough to always allow the active device to drop some voltage.
Linear drivers may be placed between the source and the regulated load (a series regulator), or may be placed in parallel with the load (shunt regulator). Simple linear regulators may for example only contain a Zener diode and a series resistor, whereas more complicated regulators include separate stages of voltage reference, error amplifier and power pass element. An emitter follower stage can be used to form a simple series voltage regulator.
The measured efficiency profiles of one example of linear LED driver at various DC bus voltages are shown in FIG. 2. The driven LED arrangement for example comprises a configuration of two parallel strings of two series LEDs giving a 20 W LED load.
The D axis stands for dimming level and the E axis stands for efficiency. FIG. 2 shows three different input bus voltages, 200V in plot 20, 210V in plot 22 and 220V in plot 24, and it shows the efficiency as a function of the dimming level. From FIG. 2, it is clear that the efficiency of a linear LED driver is very sensitive to the input-to-output voltage difference.
FIG. 3 shows the nature of the required input DC voltage VDC for efficient operation of the linear LED driver, as a function of the dimming level D. With an increase in dimming level (i.e. a reduction in the % drive level along the x-axis), the required input bus voltage across the LED and the linear driver is reduced; this is due to natural dependence of the LED string voltage on the LED current level. In particular, the LED string voltage decreases with a decrease in LED current. With a fixed DC grid voltage, the efficiency of the linear LED driver falls dramatically compared to a more efficient switch mode LED driver.
The lower efficiency of the linear driver can be addressed by providing an adaptive DC grid voltage as disclosed in WO2014/080337. A problem with this solution is that the efficiency figures deteriorate for the complete lighting system as many of them will not operate at their maximum efficiency due to variation in LED characteristics due to variation in manufacturing process from one batch to another. To avoid this problem, binning can be employed. However, this will also not be sufficient to take care of temperature and aging variations among large numbers of luminaires.
An illustration of such variations from luminaire to luminaire is shown in FIG. 4, which shows different curves of voltage versus dimming level for three different luminaires, L1, L2 and L3. In a large installation, where sets of luminaires may be connected in daisy chain mode, each luminaire experiences different voltage inputs due to cable resistance, and the DC bus experiences 100 Hz ripple from the output of a DC controller switch box. These practical issues do not yield very high efficiency for linear LED drivers at all possible dimming levels. Therefore, there is a need for a solution which enables linear LED drivers to operate at very high efficiency (such as greater than 96%) over the full dimming range i.e. from 100% to 10%, and without requiring characterization of the LEDs using binning.
Known active driver solutions need components of rated capacity and thus higher foot print and higher cost.
U.S. Pat. No. 8,710,752 discloses a system which drives multiple strings of LEDs, in which an optimal current level for each string is determined. It aims to reduce the size of the LED driver. The circuit combines a boost converter and a linear converter.